1. Field of the Invention
The present invention generally relates to a photodiode, and more specifically, to a silicon on insulator (SOI) photodiode, which has improved speed and differential isolation when used as a differential detector, and a method of forming such a photodiode.
2. Description of the Related Art
Photodiode detectors are often used in integrated optoelectronic circuits for noise rejection. Commonly assigned U. S. patent application Ser. No. 09/209,433 to Rogers et al. (hereinafter xe2x80x9cRogersxe2x80x9d) discloses a photodiode structure for use as a detector which employs lateral PIN diodes (i.e., pn junction devices with lightly doped (near intrinsic) regions placed between the typical diode p- and n-type regions) grown in deep trenches in bulk silicon. Such structures are known as xe2x80x9cbulk trench detectors.xe2x80x9d
As shown in FIG. 1(a), the bulk trench photodiode detector 100 disclosed by Rogers is composed of an array of long, deep trench structures 110 which alternate from p-type doping 110a to n-type doping 110b, thereby forming a lateral PIN diode structure. As shown in FIG. 1(b), this means that one terminal of the diode is electrically connected to the substrate, for example, the p-terminal (anode) for p-substrates, as used by typical analog and mixed signal processes.
However, in the Rogers photodiode, the terminal which is connected to the substrate will be exposed to the substrate noise and low substrate impedance associated therewith (e.g., Rs in FIG. 1(b) denotes the impedance to substrate, typically a few 10s of ohms). Therefore, if this photodiode structure is employed as a differential detector 200 as shown in FIG. 1(b), substrate noise will be injected directly into the preamplifier 210 from the diode terminal which is connected to the substrate 220 and the impedance of that substrate node will not be the same as that of the other terminal of the diode 230. An imbalanced impedance is, therefore, created at the input. Therefore, this photodiode can operate as a single-ended detector but not as a differential detector which is preferred.
Another disadvantage of this photodiode structure is that it collects photoelectrons (i.e., carriers) 120 generated below the trench depth (i.e., beneath the volume occupied by the trench electrodes) where the diminishing electric fields will sweep the carriers to appropriate device terminals with less speed than the carriers generated between the trenches, resulting in a lower bandwidth than if the carriers are collected only from the region between the trenches.
It is, therefore, an object of the invention to provide an SOI trench photodiode structure which may be electrically isolated from the bulk substrate in an optoelectronic circuit and which does not collect carriers below the trenches and may, therefore, be utilized effectively as either a single-ended detector or as a differential detector to provide higher bandwidth and flatter frequency response in optoelectronic circuits.
In a first aspect, a semiconductor photodiode is provided, including a substrate, a wafer formed on the substrate and having a silicon layer and an insulating layer, a plurality of alternating p-doped trenches and n-doped trenches formed in the silicon layer. The photodiode has a silicon epilayer which is preferably about 8 to 15 microns thick for 850 nm signals and, for reflection, an insulator layer which is about xc2xc wave thick at the signal wavelength, or about 212 nm thick for 850 nm light. The photodiode further has a set of p-doped trenches and set of n-doped trenches which are interdigitated and alternated. These trenches are also as deep as the silicon epilayer and are filled with polysilicon.
In another aspect, the photodiode has an isolation trench formed in the silicon layer and surrounding the p-doped and n-doped trenches. The trench is bounded at the bottom by the surface of the insulator layer (i.e., the trench is as deep as the silicon epilayer). In addition, the trench may be doped with either p-type or n-type dopants and filled with polysilicon or another conducting substance. The trench isolates the photodiode from other devices so that the photodiode may be used effectively as a differential detector.
In another aspect of the invention, the SOI photodiode described above is incorporated into an integrated circuit as a differential detector.
In another aspect of the invention, the isolation trench is not doped but is filled with an oxide which further isolates the photodiode.
In another aspect of the invention, the insulator layer of the SOI wafer has a dielectric reflecting stack to reflect light that has penetrated the silicon epilayer back into the trenches thereby increasing responsivity.
In another aspect of the invention, the photodiode is tilted at an angle with respect to an incoming light beam. The tilting increases the path length along which light enters the photodiode, increasing responsivity.
In another aspect of the invention, incoming light is refracted into the photodiode by placing a prismatic cover on top of the photodiode which also increases the path length along which light enters the photodiode, increasing responsivity.
In another aspect of the invention, the surface of the silicon epilayer of the SOI wafer is etched, using e.g. KOH, to form either prismatic or tetrahedral features which increases the path length along which light enters the photodiode, increasing responsivity.
In another aspect of the invention, the insulating layer on the SOI wafer is made thicker than xc2xc wavelength of the signal light and is used as a slab waveguide. Light is coupled into the insulating layer from the edge of the device, and a diffraction grating directs light upward into the active photodiode layer.
In another aspect of the invention, the photodiode device is provided without an isolation trench. This particular structure cannot operate as a differential detector but provides a high speed single-ended detector.
In yet another aspect of the invention, a method is provided for forming the SOI trench photodiode according to its several embodiments.
With its unique and unobvious features, the present invention provides a high speed photodiode that can be electrically isolated from the bulk substrate and, therefore, insensitive to substrate currents injected by other circuits fabricated in the epilayer. In addition, the inventive photodiode does not collect carriers below the trenches and is, therefore, faster than coventional photodiodes and has a flatter frequency response to lower frequencies, thereby reducing inter-symbol interference.